This invention relates to solid state focal planes for providing imaging by detecting the light emanating from an imaged scene.
The advanced imaging systems which have been developed in the infrared detection art incorporates a focal plane which is an integration of a large array of detectors with appropriate signal processing electronics. In some tactical and strategic target acquisition or surveillance applications, these arrays must be provided in a high density format, with two basic types of such infrared focal planes, monolithic and hybrid, available in designing an imaging system. A monolithic focal plane is fabricated with the multiplexer as an integral part of the detector structure, while the photodetector array and the signal multiplexer of a hybrid focal plane are produced separately, then joined together using an advanced interconnection technology.
Whether the monolithic or hybrid configuration is chosen, the focal plane must accomplish the complementary functions of photon detection including prefiltering of the optical signal, and signal multiplexing. In operation, the focal plane is irradiated with infrared background and signal energy. This optical signal is filtered and collected by the detectors, with the resulting electrical signal produced by the detectors then being coupled to the multiplexer through interfacing electronics. In this procedure, some signal processing, such as background suppression, is sometimes required for conditioning the incoming signal so that the multiplexer can be operated effectively. The output of the multiplexer will then provide a video signal which contains all the scene information within the field of view of the focal plane.
Although a number of approaches are available, the charge coupled device (CCD) has emerged as the preferred multiplexer for such focal planes due to the low noise characteristics of this device. The CCD approach to infrared focal plane multiplexing is based on the charge coupling concept, namely, the collective transfer of all the mobile charge contained within a semiconductor storage element to a similar, adjacent storage element by the external manipulation of voltages. A typical n-channel CCD consists of a p-type silicon substrate with a silicon dioxide insulating layer on its surface and an array of conducting electrodes deposited on the surface of the insulator. When a periodic waveform, known as the clock voltage, is applied to the electrode, some of the electrons in the vicinity of each electrode will form a discrete packet of charge and move a distance of one charge coupled element, or unit cell, for each full clock cycle. The packets of electron charge are thus transferred as a result of the continuous lateral displacement of the local potential wells created by the clock voltage.
In addition to its utility as a multiplexing device, the charge coupled concept may also be employed to achieve image sensing directly. Imaging with charge coupled devices has been an area of intense activity since the charge coupling technique was first developed, and imagers operating in the visible spectrum with full television resolution have been demonstrated. If an array of potential wells, such as those formed by a CCD is provided, photoemitted electrons will fill the wells to a level corresponding to the amount of light in their vicinity. These packets of electrons which are generated by the light can be transferred to a point of detection and converted to an electrical signal representing the optical image which is incident on the device.
Although such imaging devices have been investigated with a variety of gate structures, channel types, and chip layouts, most of these imagers use silicon as the photon absorbing material, thereby limiting their usefulness as infrared imagers to wavelengths less than approximately 1.1 .mu.m. Considerably interest exists, however, in imagers sensitive to longer wavelength infrared radiation. Imagers responsive in the 2-3 .mu.m range, for example, are useful in military applications for viewing high contrast scenes, such as jet and rocket plumes. Furthermore, devices responsive to higher wavelength radiation can image 300.degree. K. scenes using emitted thermal radiation, and are of interest for industrial and medical uses as well as in military applications.
One way in which silicon-based devices may be employed for imaging tasks in the medium and long wavelength infrared range is through the use of the Schottky barrier concept. A simple Schottky barrier device consists of a metal layer which has been evaporated onto a semiconductor wafer through an opening in an overlying insulator layer. This device exhibits electrical characteristics which are similar to the p-n junction, its properties depending upon the barrier height at the metal-semiconductor interface in much the same way that the characteristics of the p-n junction depend upon the bandgap. The barrier height is a function of the metal which is selected and the choice and polarity of the semiconductor, but is nearly independent of the doping applied to the semiconductor. A reverse-biased Schottky barrier diode will generate a dark current resulting from the collection by the metal of minority carriers thermally generated in the semiconductor and from the thermal excitation of majority carriers in the metal over the barrier into the semiconductor. Since the barrier height for the infrared spectral range of interest is less than half the bandgap of silicon, the latter process will dominate.
The Schottky barrier device operates as a photoconductor by absorbing light in the metal. The Schottky electrodes may be either metals or metal silicides, the latter being formed by a solid state reaction. A potential barrier will exist between the metal or silicide and the silicon substrate, so that infrared photons may pass through the silicon and be absorbed at the Schottky electrode, resulting in the excitation of carriers which are then internally emitted over the Schottky barrier into the silicon. The quantum of efficiency for this mechanism is relatively low, but the response extends to photon energies as low as the barrier height, a value which can be considerably lower than the bandgap. Since the spectral yield in such a device depends almost entirely on the absorption process in the metal and the emission of majority carriers over the barrier, its sensitivity is nearly independent of such parameters as semiconductor doping and minority carrier lifetime, thereby eliminating some of the major sources of nonuniformity in conventional semiconductors.
The use of silicon monolithic processing technology in the fabrication of Schottky retinas can lead to good photoresponse uniformities with significant reductions in the cost and complexity of a thermal energy system. The Schottky barrier detector techniques which have been developed in the prior art, however, are not adaptable to some applications because of two constraints: the necessity for a large (typically 80.times.160 .mu.m) cell size and the characteristically low "fill factor" (which may be defined as the percentage of light detecting area relative to the total focal plane area) of approximately 15-30%. The large cell size makes larger diameter optics necessary in order to collect enough signal, while the low fill factor impacts both the amount of signal collected and aliasing, i.e., the capability of the focal plane to resolve image details of a particular size. Consequently a focal plane architecture which could eliminate these limitations of the Schottky barrier detector would be welcome in the infrared imaging art and would broaden the potential applications for such detectors.